This invention relates to the field of geophysical prospecting and, more particularly, to a method for generating seismic energy for seismic surveys.
In the oil and gas industry, geophysical prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon deposits. Generally, a seismic energy source is used to generate a seismic signal that propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different acoustic impedances). The reflections are recorded by seismic detectors located at or near the surface of the earth, in a body of water, or at known depths in boreholes, and the resulting seismic data may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations.
U.S. Pat. No. 3,744,021 to Todd discloses the firing of low energy shots for shallow, high resolution profiling in combination with high energy shots for deep seismic profiling. The method only allowed for a small overlap of shallow and deep profiling recording cycles, merely maximizing the number of shots in a given period of time while minimizing interference. U.S. Pat. No. 5,973,995 to Walker and Lindtjeorn discloses a method for simultaneous recording of deep and shallow profiling data. Their main objective was to use different cables in one and the same shooting configuration.
U.S. Pat. No. 4,168,485 to Payton, et al, teaches a full simultaneous signal generation method. This patent implements orthogonal pseudorandom sequences for vibratory sources allowing for the separation of the source signals during the correlation process. Experiments with pseudo random firings of airguns have also been conducted, however no global successes have been reported. Others have experimented with phase encoding of vibratory sources. Other patents attempting full simultaneous signal generation include U.S. Pat. No. 4,715,020 to Landrum and U.S. Pat. No. 5,822,269, to Allen. The problem with these types of encoding methods is that harmonic distortion is not rejected or is only partly rejected.
U.S. Pat. No. 4,159,463 to Silverman describes the use multiple vibrators, repeatedly vibrating at stationary locations, generating opposite polarity sweeps in encoded sequences. However, Silverman does not include the use of vibrators for firing a single shot or sweep at one set of locations while generating other polarity changing sweeps at another set of locations.
U.S. Pat. No. 5,721,710 to Sallas teaches a generalized method for the simultaneous use of an arbitrary number of vibrators, sweeping a specified number of times in constant geometry. In this method, the separation of sources is achieved through the repeated inversion of two-dimensional (source versus shot) matrices at constant frequencies.
A general limitation when using pseudo-random sequences and sweep signals is the length of the energy emission, rendering the method less attractive for dynamic (marine) recording. Methods that are applicable to explosive and implosive types of sources are limited. U.S. Pat. No. 5,924,049 to Beasley and Chambers teaches a processing method to separate the signals from different sources when fired simultaneously from two ends of recording cable(s). The method is not suitable for the simultaneous recording of signals arriving from approximately the same direction.
A method disclosed by U.S. Pat. No. 4,953,657 to Edington discloses use of a suite of time delay differences between sources. To enhance the signal from a particular source, the corresponding signals are aligned and stacked. The contributions from the other source(s) are not aligned and do not stack to full strength. The remaining undesired energy is further attenuated in the frequency domain.
The high cost of seismic acquisition necessitates that compromises in the field be made, both on land and offshore. The common practice is to acquire data at a low but still acceptable areal density of surface locations. On land both the source and the receiver deployment may be less than optimal while in the marine environment the source deployment is routinely compromised and often lower than desirable. Often, data quality seems initially acceptable for the intended purpose, such as reconnaissance, new field exploration, wildcat drilling, etc. However, when more detailed studies like hydrocarbon identification and reservoir characterization are needed at a later stage, the data quality proves insufficient. In both land and marine environments there is a compelling case for the efficient acquisition of seismic data at a denser grid of locations. The use of multiple sources firing simultaneously into the same recording system is an attractive option to increase the field efforts at relatively low incremental cost. Simultaneous firing is particularly economical when additional sources can easily and cheaply be deployed, such as vibrator groups on land and airgun arrays in a marine situation. Unfortunately, the separation of the information pertaining to the individual sources may be cumbersome and/or imperfect.
It would be desirable to have a method of simultaneous shooting with impulsive sources. The present invention satisfies that need.
A method of seismic surveying using a plurality of simultaneously recorded seismic energy sources. An activation sequence for each of said plurality of seismic energy sources may be determined such that energy from separate seismic source positions may be recorded simultaneously and seismic energy responsive to individual seismic sources separated into separate source records. The seismic sources are activated using an activation sequence, the recorded seismic energy in the shot recordings may be separated into source recordings responsive to individual seismic sources. The source records may be derived from the shot records using a combination of shot record summations, inversions and filtering.
The present invention offers embodiments for simultaneous source separation applicable to both marine and land environments. One embodiment utilizes source signals coded with positive and negative polarities, but without the restriction of stationary locations and without the restriction of vibratory sources. Another embodiment utilizes source signals with time-delays between source activation times. These embodiments achieve enhancement of the desired source energy through the alignment and combination of the coded signals. In the source discrimination process, an equal amount of data as recorded and of data, polarity changed in processing, may be used. This aspect allows that undesired source energy and distortion energy are effectively cancelled, which are advantages over the prior art.
One embodiment of the present invention described in this disclosure is based primarily on source polarity encoding, while another embodiment is based primarily on source time shift encoding. It should be noted here that in the first part of the detailed description, the source signal has been simplified to a spike representation, or in other words it has been deconvolved for the direct source signature. This implies that the schematic data shown in the relevant figures should be convolved with the direct source signature to obtain the actual responses.